EP2749664B1 - Steel oil well pipe having excellent sulfide stress cracking resistance - Google Patents

Steel oil well pipe having excellent sulfide stress cracking resistance Download PDF

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Publication number
EP2749664B1
EP2749664B1 EP12826245.8A EP12826245A EP2749664B1 EP 2749664 B1 EP2749664 B1 EP 2749664B1 EP 12826245 A EP12826245 A EP 12826245A EP 2749664 B1 EP2749664 B1 EP 2749664B1
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content
steel
oil
residue
steel pipe
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German (de)
English (en)
French (fr)
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EP2749664A1 (en
EP2749664A4 (en
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Atsushi Soma
Tomohiko Omura
Yuji Arai
Mitsuhiro Numata
Toru Takayama
Masanao Seo
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Definitions

  • the present invention relates to a steel pipe and, in particular, to an oil-well seamless steel pipe.
  • Oil-well steel pipes are used as a casing or a tubing for an oil well or a gas well.
  • An oil well and a gas well are hereinafter referred to simply as an "oil well".
  • oil well pipes are required to have a higher strength.
  • oil well pipes of 80 ksi grade having a yield stress of 80 to 95 ksi, that is, 551 to 654 MPa
  • 95 ksi grade having a yield stress of 95 to 110 ksi, that is, 654 to 758 MPa
  • oil well pipes of 110 ksi grade having a yield stress of 110 to 125 ksi, that is, 758 to 861 MPa.
  • SSC sulfide stress cracking resistance
  • JP2001-131698A Patent Document 1
  • JP2004-332059A Patent Document 2
  • JP2001-73086A Patent Document 3 propose techniques to improve the SSC resistance by controlling nonmetallic inclusions.
  • Patent Document 1 discloses particulars described below.
  • the Ti nitride coarsely formed acts as a pitting starting point. Generation of pitting induces SSC. Therefore the Ti nitride is made finer. In this case, the SSC resistance of the steel is improved.
  • Patent Document 2 discloses particulars described below.
  • Nb-based inclusions satisfying the following expression are contained at a rate of ten or more particles per 1 mm 2 sectional area.
  • a NB is the length of the major axis ( ⁇ m) of a Nb-based inclusion
  • b NB is the content (mass%) of Nb in the Nb-based inclusion.
  • Nb-based inclusions having a NB smaller than 1 ⁇ m are ignored. In this case, the occurrence of pitting in the low alloy steel is suppressed and the SSC resistance is improved.
  • Patent Document 3 discloses particulars described below.
  • the chemical composition of a steel satisfies the following expression: 1 + 4.3 ⁇ V + 3.4 ⁇ Nb + 2.5 ⁇ Ti / 7.8 Cr ⁇ Mo > 1
  • inclusions are removed at the time of casting by causing floating of the inclusions with a tundish heater in order to suppress coarse Ti nitride.
  • floating and removal of Ti nitride at the time of casting is somewhat difficult in actual operation. Further, it is said that performing only floating and removal of Ti nitride is insufficient for suppressing the generation of pitting.
  • Patent Document 2 only the pitting resistance of an as-rolled steel member is evaluated. Patent Document 2 does not completely clarify what effect is obtained with respect to the SSC resistance relating closely to the strength of a product.
  • the content of one of the elements Cr and Mo is reduced as low as possible while the Nb content and/or the Ti content is increased. This may in some cases cause unstable hardenability. Further, SSC due to pitting, resulting from Nb-based inclusions and/or Ti-based inclusions, was not sufficiently considered.
  • JP 2001-172739 discloses a steel for oil well use with a composition containing 0.15 to 0.30% C, 0.05 to 1.0% Si, 0.10 to 1.0% Mn, ⁇ 0.025% P, ⁇ 0.005% S, 0.1 to 1.5% Cr, 0.1 to 1.0% Mo, 0.003 to 0.08% Al, ⁇ 0.008% N, 0.0005 to 0.010% B and ⁇ 0.008% Ca+O(oxygen) and moreover containing one or more kinds among 0.005 to 0.05% Ti, ⁇ 0.05% Nb, ⁇ 0.05% Zr and ⁇ 0.30% V, and in which the properties of inclusions in the steel satisfy the following inequalities (a) and (b): the maximum length Lx ⁇ 80 ⁇ m ... (a), and the number Kz of the ones with the grain sizes of ⁇ 20 ⁇ m ⁇ 10 pieces/100 mm 2 ... (b).
  • EP-A1-2,133,443 discloses a low alloy steel for oil country tubular goods with a yield strength between 654 MPa and 757 MPa, and possessing excellent resistance to HIC and SSC in highpressure hydrogen sulfides environment, and comprising, by mass %: 0.10 to 0.60% C; 0.05 to 0.5% Si; 0.05 to 3.0% Mn; 0.025% or less P; 0.010% or less S; 0.005 to 0.10% Al; 0.01% or less 0 (oxygen); 3.0% or less Cr; and 3.0% or less Mo, wherein the amount of Cr and Mo content is 1.2% or more, with the balance being Fe and impurities, and the number of nonmetallic inclusions whose major axis is 10 ⁇ m or more is 10 per square millimeter in the inspected cross section.
  • JP 2001-073086 discloses a steel with a composition containing 0.15-0.35% C, 0.1-1.5% Si, 0.1-1.0% Mn, ⁇ 0.03% P, ⁇ 0.01 0% S, 0.001-0.1% sol. Al, 0.0001-0.0030% B, and ⁇ 0.0070% N and satisfying inequality (1 +4.3x[V]+3.4x [Nb]+2.5x[Ti])/(7.8x[Cr]x[Mo]) ⁇ 1 (where the terms in brackets represent weight percentages of respective alloying elements).
  • JP 2001-131698 discloses a steel containing, by mass, 0.2 to 0.35% C, 0.05 to 0.5% Si, 0.1 to 1% Mn, ⁇ 0.025% P, ⁇ 0.01% S, 0.1 to 1.2% Cr, 0.1 to 1% Mo, 0.005 to 0.1% Al, 0.0001 to 0.01% B, 0.005 to 0.5% Nb, ⁇ 0.005% N, ⁇ 0.01% O (oxygen), ⁇ 0.1% Ni, Ti: 0.005 to 0.03% and also ⁇ 0.00008/N%, 0 to 0.5% V, 0 to 1% W, 0 to 0.1% Zr, 0 to 0.01% Ca, and the balance Fe with impurities, in which the number of TiN with a diameter of ⁇ 5 ⁇ m is ⁇ 10 pieces per mm 2 of the cross section and having yield stress of ⁇ 758 MPa.
  • EP-A1-1,785,501 discloses a low alloy steel for oil well pipes excellent in sulfide stress cracking resistance containing, by mass %, C: 0.2 to 0.35%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%, P: 0.025% or less, S: 0.01% or less, Al: 0.005 to 0.10%, Cr: 0.1 to 1.0%, Mo: 0.5 to 1.0%, Ti: 0.002 to 0.05%, V: 0.05 to 0.3%, B: 0.0001 to 0.005%, N: 0.01% or less, O (oxygen): 0.01% or less, Nb: 0 to 0.1 %, Ca: 0 to 0.01 %, Mg: 0 to 0.01% and Zr: 0 to 0.1 %, in which a half-value width H and a hydrogen diffusion coefficient D (10 -6 cm 2 /s) satisfy the following equation (1): 30 H + D ⁇ 19.5
  • EP-A1-1,712,651 discloses a high-strength seamless steel pipe for oil wells excellent in sulfide stress cracking resistance which comprises, on the percent by mass basis, C: 0.1 to 0.20%, Si: 0.05 to 1.0%, Mn: 0.05 to 1.0%, Cr: 0.05 to 1.5%, Mo: 0.05 to 1.0%, Al: 0.10% or less, Ti: 0.002 to 0.05% and B: 0.0003 to 0.005%, with a value of equation "C + (Mn/6) + (Cr/5) + (Mot3)" of 0.43 or more, with the balance being Fe and impurities, and in the impurities P: 0.025% or less, S: 0.010% or less and N: 0.007% or less.
  • US 2006/018783 discloses a steel product for oil country tubular good according to the invention comprises, in mass %, 0.10% to 0.35% C, 0.10% to 0.50% Si, 0.10% to 0.80% Mn, up to 0.030% P, up to 0.010% S, 0.30% to 1.20% Cr, 0.20% to 1.00% Mo, 0.005% to 0.40% V, 0.005% to 0.100% Al, up to 0.0100% N, up to 0.0010% H, 0 to 0.01% Ca, 0 to 0.050% Ti, 0 to 0.050% Nb, and 0 to 0.0050% B, and the balance of Fe and impurities.
  • EP-A1-1,790,748 discloses a steel for steel pipes which comprises, on the percent by mass basis, C: 0.2 to 0.7%, Si: 0.01 to 0.8%, Mn: 0.1 to 1.5%, S: 0.005% or less, P: 0.03% or less, Al: 0.0005 to 0.1%, Ti: 0.005 to 0.05%, Ca: 0.0004 to 0.005%, N: 0.007% or less, Cr: 0.1 to 1.5%, Mo: 0.2 to 1.0%, Nb: 0 to 0.1 %, Zr: 0 to 0.1%, V: 0 to 0.5% and B: 0 to 0.005%, with the balance being Fe and impurities, in which non-metallic inclusions containing Ca, Al, Ti, N, O, and S are present, and in the said inclusions (Ca%)/(Al%) is 0.55 to 1.72, and (Ca%)/(Ti%) is 0.7 to 19 can be used as a raw material for oil country tubular goods, being used at a greater depth
  • JP H09-67624 discloses a steel pipe having a composition consisting of, by weight 0.15-0.30% C, 0.05-1.00% Si, 0.20-1.50% Mn, ⁇ 0.02% P, 0.3-1.5% Cr, 0.10-1.00% Mo, 0.01-0.07% Al, ⁇ 0.015% N, ⁇ 0.0015% S, 0.0002-0.0010% Ca, one or more kinds among 0.01-0.05% Nb, 0.01-0.05% Ti, 0.01-0.10% V, and 0.001-0.005% B, and the balance Fe.
  • JP H07-197125 discloses a steel composition by wt.%, 0.15-0.4 C, 0.1-1 Si, 0.3-1 Mn, ⁇ 0.1-1.5 Cr, 0.3-1 Mo, 0.0005-0.003 B, 0.01-0.1 Al, 0.003-0.01 N, ⁇ 0.015 P, ⁇ 0.005 S, and further, one or more kinds of 0.01-0.05 V, 0.01-0.05 Nb and 0.01-0.03% Ti and the balance Fe.
  • An objective of the present invention is to provide an oil-well steel pipe having excellent SSC resistance.
  • the oil-well seamless steel pipe according to the present invention consists of, by mass percent, C: 0.15 to 0.35%, Si: 0.1 to 0.75%, Mn: 0.1 to 1.0%, Cr: 0.1 to 1.7%, Mo: 0.1 to 1.2%, Ti: 0.01 to 0.05%, Nb: 0.010 to 0.030%, Al: 0.01 to 0.1%, N: 0.001 to 0.007%, V: 0.01 to 0.50%, and Ca: 0.0003 to 0.0050%, optionally B: at most 0.0050% the balance being Fe and impurities, P, S and O in the impurities being P: at most 0.03%, S: at most 0.01%, and O: at most 0.01%, wherein the Ti content and the Nb content in a residue obtained by using a bromine-methanol extraction satisfy equation (1): 100 ⁇ Nb / Ti + Nb ⁇ 27.5.
  • the oil-well seamless steel pipe has a prior austenite grain size number of 7.5 or larger as measured in accordance with ASTM E112, the Ti content and the Nb content in a residue obtained by bromine-methanol extraction are obtained by the steps of: taking a sample with a weight of 1 g from the oil-well seamless steel pipe, immersing the sample in a methanol solution containing 10 volume percent of bromine to be dissolved, filtering the methanol solution in which the sample is dissolved using a filter having a pore size of 0.2 ⁇ m, dissolving the filter using H 2 SO 4 to extract residue attached to the filter, placing the residue and a mixed acid containing HCl, HNO 3 and H 2 O in a ratio HCl : HNO 3 : H 2 O of 1 : 1 : 8 in a hermetically sealed container, heating the container at 220 °C for 8 hours to decompose the residue in the container, followed by cooling of the decomposed residue by unforced cooling to obtain an acid solution, where
  • the oil-well steel pipe according to the present invention has excellent SSC resistance.
  • the above-described oil-well steel pipe may contain B: 0.0001 to 0.0050%.
  • Figure 1 is a graph showing the relationship between the Ti content and the Nb content in a residue extracted by using a bromine-methanol solution and the number of coarse particles of carbo-nitrides and nitrides in steel (coarse inclusions, i.e. those having an inclusion length of 20 ⁇ m or more).
  • the present inventors examined and studied the SSC resistance of oil-well steel pipes to obtain the findings described below.
  • Figure 1 is a graph showing the relationship between an F1 value in a residue obtained by using bromine-methanol extraction and the number of particles of carbo-nitrides and nitrides having an inclusion length of 20 ⁇ m or more.
  • Figure 1 was a result of a method described below. As shown in examples described later, a plurality of steel pipes having a plurality of chemical compositions were produced. To determine the F1 value from each steel pipe, a specimen (1 g) for bromine-methanol extraction was sampled. Further, to determine the number of particles of carbo-nitrides and nitrides in the steel, a specimen for microscopic observation was sampled from an arbitrary position in each steel pipe.
  • the specimen for bromine-methanol extraction was dissolved by using a methanol solution containing 10 volume percent (vol%) of bromine (hereinafter referred to as "bromine-methanol solution”) to obtain a residue. More specifically, the specimen for bromine-methanol extraction was immersed in the bromine-methanol solution and dissolved. The bromine-methanol solution in which the specimen was dissolved was filtered by using a filter. A residue attached to the filter. The filter to which the residue attached was dissolved by using H 2 SO 4 to obtain the residue. The residue substantially contained carbo-nitrides, nitrides and oxides.
  • the residue was decomposed by a pressurized acidolysis method.
  • the residue and an acid for decomposition were contained in a polytetrafluoroethylene (PTFE) container.
  • PTFE polytetrafluoroethylene
  • the container was hermetically sealed and heated at 220 °C for 8 hours to decompose the residue in the container.
  • the decomposed residue was left to cool.
  • the resulted acid solution was then fixed to a certain volume and the Ti content and the Nb content were analyzed by radiofrequency inductive coupled plasma (ICP) emission spectrochemical analysis method.
  • ICP radiofrequency inductive coupled plasma
  • the above-described F1 value was determined based on the obtained Ti and Nb contents. Most of the carbo-nitrides and nitrides formed in the above-described chemical composition are either a Ti-based inclusion or an Nb-based inclusion. Therefore, the F1 value is thought to be a measure indicating the proportion of the Nb content in the carbo-nitrides and nitrides.
  • the number of particles of the carbo-nitrides and nitrides in the steel were determined by a method described below.
  • the surface of the specimen was first polished.
  • An arbitrary region (100 mm 2 ) in the polished surface was selected.
  • the selected region was observed through a 200 to 1000 ⁇ optical microscope.
  • the size of each of the carbo-nitrides and nitrides was determined by a method described below.
  • the length of the major axis of each of the carbo-nitrides and nitrides was defined as "inclusion length" to be used as a measure for the size of the carbo-nitrides and nitrides.
  • inclusion length When the distance between inclusions (carbo-nitrides and nitrides) adjacent to each other is 40 ⁇ m or less, the inclusions were regarded as one continuous particle of inclusions.
  • the number of particles of the carbo-nitrides and nitrides having an inclusion length of 20 ⁇ m or more (hereinafter referred to as "coarse inclusions or coarse particles of inclusions") in the region was determined.
  • Figure 1 showing the relationship between the number LN 100 of coarse particles of inclusions and F1 was prepared.
  • the number LN 100 of coarse particles of inclusions was much smaller when the F1 value was 27.5 or less than when the F1 value was larger than 27.5. Therefore, excellent SSC resistance can be obtained when the F1 value is 27.5 or less.
  • the oil-well steel pipe according to the present invention has a chemical composition described below.
  • Carbon (C) increases hardenability and increases the strength of steel.
  • the C content is excessively high, the susceptibility of steel to quench cracking is increased. Further, the toughness of steel is lowered. Accordingly, the C content is 0.15 to 0.35%.
  • the C content is preferably higher than 0.15%, more preferably equal to or higher than 0.20%, further preferably equal to or higher than 0.22%.
  • the C content is preferably lower than 0.35%, more preferably equal to or lower than 0.33%, further preferably equal to or lower than 0.32%.
  • the Si content deoxidizes steel. On the other hand, if the Si content is excessively high, the toughness and hot workability of steel are lowered. Accordingly, the Si content is 0.1 to 0.75%. Regarding the lower limit of the Si content, the Si content is preferably higher than 0.1%, more preferably equal to or higher than 0.15%, further preferably equal to or higher than 0.20%. Regarding the upper limit of the Si content, the Si content is preferably lower than 0.75%, more preferably equal to or lower than 0.50%, further preferably equal to or lower than 0.35%.
  • Mn Manganese
  • the Mn content is 0.1 to 1.0%.
  • the Mn content is preferably higher than 0.1%, more preferably equal to or higher than 0.15%, further preferably equal to or higher than 0.2%.
  • the Mn content is preferably lower than 1.0%, more preferably equal to or lower than 0.7%, further preferably equal to or lower than 0.6%.
  • Chromium (Cr) increases the hardenability and temper softening resistance of steel. Therefore Cr facilitates execution of high-temperature tempering on steel. Cr further improves the SSC resistance of steel. On the other hand, if the Cr content is excessively high, M 7 C 3 -based carbides and M 23 C 6 -based carbides are formed and the SSC resistance of steel is lowered. Accordingly, the Cr content is 0.1 to 1.7%. Regarding the lower limit of the Cr content, the Cr content is preferably higher than 0.1%, more preferably equal to or higher than 0.3%, further preferably equal to or higher than 0.4%. Regarding the upper limit of the Cr content, the Cr content is preferably lower than 1.7%, more preferably equal to or lower than 1.5%, further preferably equal to or lower than 1.2%.
  • Molybdenum (Mo) increases the hardenability and temper softening resistance of steel. Therefore Mo facilitates execution of high-temperature tempering on steel. Mo further improves the SSC resistance of steel. On the other hand, if the Mo content is excessively high, the above-described effects are saturated. Accordingly, the Mo content is 0.1 to 1.2%. Regarding the lower limit of the Mo content, the Mo content is preferably higher than 0.1%, more preferably equal to or higher than 0.2%, further preferably equal to or higher than 0.4%. Regarding the upper limit of the Mo content, the Mo content is preferably lower than 1.2%, more preferably equal to or lower than 1.0%, further preferably equal to or lower than 0.9%.
  • Ti nitride and/or Ti carbo-nitride causes crystal grains of steel to be finer.
  • boron (B) is contained, Ti suppresses the formation of B nitride. Therefore the improvement in hardenability due to B is increased.
  • the Ti content is excessively high, Ti nitride and/or Ti carbo-nitride is coarsely formed, resulting in a reduction in SSC resistance of steel. Accordingly, the Ti content is 0.01 to 0.05%.
  • the Ti content is preferably higher than 0.01%, more preferably equal to or higher than 0.011%, further preferably equal to or higher than 0.012%.
  • the Ti content is preferably lower than 0.05%, more preferably equal to or lower than 0.03%, further preferably equal to or lower than 0.025%.
  • Nb carbo-nitride and Nb nitride Niobium bonds to C and N to form Nb carbo-nitride and Nb nitride.
  • Nb and Ti or Al may further form a composite carbo-nitride. These inclusions, if they are fine, cause crystal grains to be finer.
  • the Nb content is 0.010 to 0.030%.
  • the Nb content is preferably higher than 0.010%, more preferably equal to or higher than 0.011%, further preferably equal to or higher than 0.012%.
  • the Nb content is preferably lower than 0.030%, more preferably equal to or lower than 0.020%, further preferably equal to or lower than 0.015%.
  • Aluminum (Al) deoxidizes steel. On the other hand, if the Al content is excessively high, Al-based oxides are coarsely formed to reduce the toughness of steel. Accordingly, the Al content is 0.01 to 0.1%. Regarding the lower limit of the Al content, the Al content is preferably higher than 0.01%, more preferably equal to or higher than 0.015%, further preferably equal to or higher than 0.020%. Regarding the upper limit of the Al content, the Al content is preferably lower than 0.1%, more preferably equal to or lower than 0.07%, further preferably equal to or lower than 0.05%. "Al content” referred to in the present specification denotes the content of acid-soluble Al (sol. Al).
  • the balance in the oil-well steel pipe according to the present invention is Fe and impurities.
  • impurities in the present specification denotes elements mixed from an ore or scraps used as a raw material for steel or a production process environment or the like. In the present invention, the contents of P, S, N and O as impurities are suppressed as described below.
  • Phosphorous (P) is an impurity. P segregates at grain boundaries to make the grain boundaries brittle. Therefore P lowers the toughness and SSC resistance of steel. For this reason, it is preferable to set the P content as low as possible.
  • the P content is equal to or lower than 0.03%.
  • the P content is preferably lower than 0.03%, more preferably equal to or lower than 0.02%, further preferably equal to or lower than 0.015%.
  • S Sulfur
  • Mn Mn-based sulfide
  • S causes reductions in toughness and SSC resistance of steel. For this reason, it is preferable to set the S content as low as possible.
  • the S content is equal to or lower than 0.01%.
  • the S content is preferably lower than 0.01%, more preferably equal to or lower than 0.002%.
  • N Nitrogen
  • N is an impurity. N coarsens Nb-based inclusions and/or Ti-based inclusions. The coarsened Nb-based inclusions and Ti-based inclusions cause a reduction in pitting resistance of steel and, hence, a reduction in SSC resistance. Therefore it is preferable to set the N content as low as possible.
  • the N content is equal to or lower than 0.007%.
  • the N content is preferably lower than 0.007%, more preferably equal to or lower than 0.005%.
  • N is contained at least 0.001%.
  • Oxide (O) is an impurity. O forms oxides in coarse form to reduce the pitting resistance of steel. Therefore it is preferable to set the O content as low as possible.
  • the O content is equal to or lower than 0.01%.
  • the O content is preferably less than 0.01%, more preferably equal to or lower than 0.003%, and further more preferably equal to or lower than 0.0015%.
  • the oil-well steel pipe according to the present invention contains V in place of some part of Fe.
  • V 0.01 to 0.50%
  • Vanadium (V) forms fine carbides in a tempering process to improve the temper softening resistance. Tempering at high temperatures is thereby enabled to improve the toughness and SSC resistance of steel. If even a small amount of V is contained, the above-described effect can be obtained. On the other hand, if the V content is excessively high, the above-described effect is saturated. Accordingly, the V content is 0.50% or less. Regarding the lower limit of the V content, the V content is equal to or higher than 0.01%, preferably equal to or higher than 0.03%, further preferably equal to or higher than 0.05%. Regarding the upper limit of the V content, the V content is preferably lower than 0.50%, more preferably equal to or lower than 0.2%, further preferably equal to or lower than 0.15%.
  • the oil-well steel pipe according to the present invention may further contain B in place of some part of Fe.
  • B Boron
  • B is an optional element.
  • B increases the hardenability of steel. If even a small amount of B is contained, the above-described effect can be obtained. On the other hand, if the B content is excessively high, the above-described effect is saturated. Accordingly, the B content is 0.0050% or less.
  • the B content is preferably equal to or higher than 0.0001%, more preferably equal to or higher than 0.0005%.
  • the B content is preferably lower than 0.0050%, more preferably equal to or lower than 0.0025%.
  • the oil-well steel pipe according to the present invention contains Ca in place of some part of Fe.
  • Ca Calcium
  • the Ca content is 0.0050% or less.
  • the Ca content is equal to or higher than 0.0003%, preferably equal to or higher than 0.0005%.
  • the Ca content is preferably lower than 0.0050%, more preferably equal to or lower than 0.0030%.
  • the chemical composition of the oil-well steel pipe according to the present invention may satisfy the following expression (A): 1 + 4.3 ⁇ V + 3.4 ⁇ Nb + 2.5 ⁇ Ti / 7.8 ⁇ Cr ⁇ Mo ⁇ 1.
  • the left-hand side of the above expression (A) is larger than 1 as a condition.
  • the left-hand side of the above expression may be 1 or less.
  • the Ti content and the Nb content are lower, it is easier to control the number of coarse particles of carbo-nitrides and nitrides, and the SSC resistance is improved.
  • the left-hand side of expression (A) be 1 or less. More preferably, the left-hand side of expression (A) is 0.85 or less. Further preferably, the left-hand side of expression (A) is 0.65 or less.
  • the Ti content and the Nb content in a residue obtained by means of bromine-methanol extraction satisfies expression (1).
  • carbides do not easily affect the SSC resistance.
  • coarse carbo-nitrides and nitrides cause a reduction in SSC resistance.
  • Carbo-nitrides and nitrides in fine form in the steel make crystal grains finer. Even if the number of fine particles of carbo-nitrides and nitrides is small, crystal grains are made finer to some extent.
  • the number of coarse particles of carbo-nitrides and nitrides is increased, the SSC resistance of the steel is lowered, as described above. Therefore, excellent SSC resistance can be obtained if the number of coarse particles of carbo-nitrides and nitrides is suppressed.
  • Bromine-methanol extraction is performed as described below.
  • a sample is taken from an arbitrary position in the oil-well steel pipe. The shape of the sample is not restrictively specified.
  • the weight of the sample is 1 g.
  • the sample is immersed in a methanol solution containing 10 volume percent (vol%) of bromine (hereinafter referred to as "bromine-methanol solution") to be dissolved.
  • the bromine-methanol solution in which the sample is dissolved is filtered by using a filter having a pore size of 0.2 ⁇ m (e.g., a Nuclepore filter having a pore size of 0.2 ⁇ m).
  • a residue attaches to the filter.
  • the filter to which the residue has attached is dissolved by using H 2 SO 4 to extract the residue.
  • Carbides in the steel is dissolved in the bromine-methanol solution. Therefore the residue substantially contains carbo-nitrides, nitrides and oxides.
  • the residue is decomposed, by a pressurized acidolysis method.
  • the residue and an acid for decomposition are contained in a container (e.g., a PTFE container).
  • the container is hermetically sealed and heated at 220 °C for 8 hours.
  • An alkali fusion method not in accordance with the present invention also can be used to decompose the residue instead of the pressurized acidolysis method mentioned above.
  • the residue and a fusing agent are put in a platinum crucible.
  • the fusing agent is, for example, LiBO 2 .
  • the platinum crucible is heated at 1000°C in an electric furnace to decompose the residue in the platinum crucible.
  • the residue can be easily dissolved in an acid solution.
  • the residue decomposed by the pressurized acidolysis method is cooled by unforced cooling.
  • the resulted acid solution is then fixed to a certain volume and the Ti content and the Nb content are analyzed by an ICP emission spectrochemical analysis method.
  • F1 is a measure indicating the Nb concentration in the carbo-nitrides and nitrides.
  • the number of coarse particles of inclusions is preferably 35 particles/100 mm 2 or less.
  • the oil-well steel pipe in the present invention has an yield strength of 654 MPa or more.
  • Yield strength designates 0.2% yield stress. More preferably, the yield strength of the oil-well steel pipe is 758 MPa or more.
  • the oil-well steel pipe according to the present invention has high strength. Therefore, if the tensile strength is excessively high relative to the yield strength, the SSC resistance is lowered. Accordingly, a preferable yield ratio is 87.0% or more.
  • a prior austenite grain size number of the oil-well steel pipe according to the present invention is 7.5 or larger.
  • the prior austenite grain size number referred to in the present specification is measured in accordance with ASTM E112. When the prior austenite grain size number is smaller than 7.5, the toughness and SSC resistance of the steel are lowered.
  • primary refining is performed on pig iron with a converter or an electronic furnace. Further, secondary refining is performed on molten steel that has undergone primary refining, and alloying elements are added to the molten steel. Molten steel having the above-described chemical composition is produced by this process.
  • the molten steel is poured into a tundish, and a slab or a round billet or the like is produced by a continuous casting process.
  • a slab or a round billet or the like is produced by a continuous casting process.
  • an ingot is produced from the molten steel by an ingot-making process.
  • the slab or the like, or the ingot is subjected to blooming to produce a round billet.
  • the temperature of the molten steel in the tundish is held at 1520°C or higher.
  • inclusions, i.e., impurities, in the molten steel coagulate and float up in the tundish. Removal of the inclusions is thus enabled.
  • the cooling rate for the casted or poured slab, round billet or the like, or ingot is 50°C/minute or higher. In this case, coarsening of the inclusions is suppressed.
  • the round billet is subjected to hot working to be formed into a hollow shell.
  • the round billet is heated in a heating furnace.
  • Hot working is performed on the round billet extracted from the heating furnace to produce a hollow shell (a seamless steel pipe).
  • a Mannesmann process is performed as hot working to produce a hollow shell.
  • piercing-rolling is performed on the round billet with a piercing machine.
  • Hot elongating is further performed on the piercing-rolled round billet with a mandrel mill, a reducer, a sizing mill or the like to form a hollow shell.
  • a hollow shell may be formed from the round billet by a different hot working process.
  • the round billet heating temperature and heating time satisfy the following expression (2): T + 273 ⁇ 20 + log t ⁇ 30600.
  • a heating temperature (°C) is substituted for T in expression (2), and a heating time (hr) is substituted for t.
  • the heating furnace is divided into a plurality of zones.
  • the heating furnace is divided into a preheating zone, a heating zone and a soaking zone, for example.
  • the zones are disposed in a row, and the round billet is heated while being moved in order of the preheating zone, the heating zone and the soaking zone.
  • the heating temperature and heating time for one of the zones may be different from those for the other zones.
  • the average of the heating temperatures of the zones is defined as the heating temperature T (°C) of the heating furnace.
  • the value of the accumulation of the heating times for the zones is defined as the heating time t (hr) of the heating furnace.
  • F2 (T + 273) ⁇ (20 + log (t)) is defined.
  • the lower limit of the value F2 is preferably equal to or larger than 28500, more preferably equal to or larger than 29200. If the value F2 is excessively small, a temperature of the round billet suitable for piercing cannot be easily reached.
  • the round billet soaking temperature (the temperature in the soaking zone) is preferably equal to or higher than 1200°C.
  • the hollow shell after hot working is cooled to normal temperature.
  • quenching and tempering are performed to produce an oil-well steel pipe.
  • the quenching temperature is equal to or higher than A C3 point.
  • tempering the tempering temperature is equal to or lower than A C1 point.
  • the hollow shell having a surface temperature equal to or higher than A C3 point after hot working may be directly subjected to quenching without being cooled to normal temperature. Further, the hollow shell after hot working may alternatively be inserted in a heating furnace immediately after the hot working and subjected to complementary heating (soaking) to a temperature equal to or higher than A C3 point. In this case, quenching is performed on the hollow shell after complementary heating. Quenching and tempering may be performed a certain number of times. More specifically, on the hollow shell on which quenching and tempering have been performed, quenching and tempering may be further performed.
  • the oil-well steel pipe produced by the above-described production process satisfies expression (1). Therefore the oil-well steel pipe has excellent SSC resistance.
  • Oil-well steel pipes having various chemical compositions were produced under various production conditions.
  • the SSC resistances of the produced oil-well steel pipes were evaluated.
  • Round billets having a diameter of 310 mm were produced by a continuous casting process using molten steels as steels A to J (230 tons).
  • the round billets were heated in the heating furnace at billet heating conditions shown in Table 2.
  • Piercing-rolling after heating was performed on the round billets in a Mannesmann process, thereby producing hollow shells.
  • Quenching was performed on the hollow shells at quenching temperatures shown in Table 2, and tempering was performed on the hollow shells at tempering temperatures shown in Table 2, thereby producing oil-well steel pipes. Quenching and tempering conditions were adjusted so that the yield strengths of the oil-well steel pipes are in the 110 ksi grade (758 to 862 MPa).
  • the outside diameters and the wall thicknesses of the produced oil-well steel pipes were as shown in Table 2.
  • a “billet heating condition” section in Table 2 contains entries of the heating temperatures (°C) and the heating times (hr) for the regions (the preheating zone, the heating zone, and the soaking zone) of the heating furnace in the hot working process.
  • a “heating temperature” section contains entries of the averages (°C) of the heating temperatures for the zones.
  • An “accumulated heating time” section contains entries of the values of the accumulations of the heating times for the zones.
  • An “F2" section contains entries of the F2 values.
  • a "heat treatment process” section in Table 2 contains entries of heat treatment processes performed on steels A to J.
  • "Off-Line QT" designates the following heat treatment process.
  • the hollow shell after hot working was cooled to normal temperature (25°C). Quenching was performed by heating the cooled hollow shell to a temperature equal to or higher than A C3 point. Tempering was performed at a temperature equal to or lower than A C1 point on the hollow shell after quenching.
  • In-Line QT designates the following heat treatment process.
  • the hollow shell after hot working was subjected to soaking (concurrent heating) at a temperature equal to or higher than A C3 point in the heating furnace without being cooled to normal temperature.
  • Quenching and tempering were performed on the hollow shell after soaking.
  • the quenching temperature was 950°C and the tempering temperature was 560°C.
  • the soaking time for soaking at the quenching temperature in each quenching in "Off-Line QT" and "In-Line QT" was 30 to 90 minutes, and the soaking time for soaking at the tempering temperature in each tempering was 20 to 100 minutes.
  • a specimen was sampled from each steel pipe.
  • the specimen had a surface perpendicular to the longitudinal direction of the steel pipe (hereinafter referred to as "observed surface").
  • the observed surface of the specimen was mechanically polished. After polishing, a prior austenite grain boundary of the observed surface was caused to appear by using a Picral etching reagent. Thereafter, the prior austenite grain size number of the observed surface was determined in accordance with ASTM E112.
  • An arched tensile test specimen was sampled from each steel pipe. A cross-sectional surface of the tensile test specimen was arched and the longitudinal direction of each tensile test specimen was parallel to the longitudinal direction of the steel pipe. A tensile test was performed in accordance with the specifications in the API standard 5CT by using the arched tensile test specimens. The yield strength YS (MPa), the tensile strength TS (MPa) and the yield ratio YR (%) of the steel pipe were determined based on the test results.
  • the Ti content and Nb content in a residue obtained by bromine-methanol extraction were determined by the above-described method. More specifically, a 1 g specimen was sampled from each steel pipe. A residue (inclusions) was obtained by using the sampled specimen and by using the above-described bromine-methanol solution. The residue was decomposed by the above-described pressurized acidolysis method. The Ti content and the Nb content in the residue were determined by the ICP emission spectrochemical analysis method. The F1 value was determined by using the Ti content and the Nb content.
  • a round bar specimen was sampled from each steel pipe.
  • the longitudinal direction of the round bar specimen was parallel to the longitudinal direction of the steel pipe.
  • the outside diameter of a parallel portion of the round bar specimen was 6.35 mm and the length of the parallel portion was 25.4 mm.
  • the SSC resistance of each round bar specimen was evaluated by a constant load test in accordance with the NACE (National Association of Corrosion Engineers) TM0177 Method A.
  • the testing bath was a 5% sodium chloride + 0.5% acetic acid aqueous solution of normal temperature in which hydrogen sulfide gas was saturated at 1 atm.
  • Each round bar specimen was immersed in the testing bath for 720 hours while being loaded with a load stress of 645 MPa.
  • the load stress was 85% of the nominal yield stress in the 110 ksi grade. After a lapse of 720 hours from starting immersion, a check was made as to whether or not any rupture was observed in the round bar specimen. When no rupture was observed in the round bar specimen, it was determined that the SSC resistance of the steel was high. When a rupture was observed in the round bar specimen, it was determined that the SSC resistance of the steel was low.
  • a “YS” section in Table 3 contains entries of yield strengths (MPa).
  • a “TS” section contains entries of tensile strengths (MPa).
  • a “YR” section contains entries of yield ratios (%).
  • An “F1” section contains entries of F1 values.
  • a “prior ⁇ grain size No.” section contains entries of prior austenite grain size Nos.
  • a “SSC resistance” section contains entries of SSC resistance evaluation test results.
  • “NF” designates a fact that no rupture was observed in the round bar specimen and excellent SSC resistance was obtained.
  • “F” designates a fact that a rupture was observed in the round bar specimen and the SSC resistance was low.
  • Steel I had an F2 value satisfying equation (2) but its Nb content was lower than the lower limit in accordance with the present invention. As a result, in steel I, crystal grains were not made sufficiently fine and a rupture was observed in the SSC resistance evaluation test.
  • Steel J had an F2 value satisfying equation (2) but its Nb content exceeded the upper limit in accordance with the present invention. As a result, a large number of coarse particles of inclusions were formed and a rupture was observed in the SSC resistance evaluation test.

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CA2843593C (en) 2018-10-02
WO2013027666A1 (ja) 2013-02-28
EA201490487A1 (ru) 2014-06-30
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AU2012297775B2 (en) 2015-06-04
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